In a vertical laser structure, there is an active region in which optical gain occurs, and light is emitted from the top or the bottom (major) surface, or both, of the laser structure. A vertical optical cavity is created in the structure by virtue of semi-reflecting optical reflector(s) located on the top or bottom surface thereof, or both.
The structure of a vertical laser can be made circularly symmetric. Therefore, a vertical laser can have the advantage of relatively low astigmatism as compared with other lasers, such as "edge" lasers in which light is emitted from a side edge of the semiconductor body.
In a Japanese patent No. 60-81888 entitled "Surface Light Emitting Laser and Manufacture Thereof" issued to H. Tanaka on July 9, 1985, a vertical-cavity semiconductor laser was disclosed in which one of the reflectors (on the bottom, non-emitting surface) was a mirror stack (plurality) of alternating in situ epitaxially-grown layers of n-type (or p-type) semiconductive Al.sub.x Ga.sub.1-x As and n-type (or p-type) semiconductive Al.sub.y Ga.sub.1-y As, with y being different from x. The thickness of each layer was selected to be equal to a quarter of the wavelength (measured in the semiconductor) of the central optical wavelength, that is, .lambda./4n, where .lambda. is the vacuum wavelength of the center of the optical distribution, and n is the refractive index. Because the dielectric constant difference between Al.sub.x Ga.sub.1-x As and Al.sub.y Ga.sub.1-y As is relatively small regardless of x and y, it requires a relatively large number of such epitaxially-grown layers in the mirror stack to achieve a desirably high overall optical reflectivity for the stack, whereby the optical stop band of the reflector is relatively small and processing time is relatively large.
In a paper entitled "TiO.sub.2 (or Si)/SiO.sub.2 Dielectric Multilayer Reflector For Surface Emitting Lasers," authored by T. Sakaguchi et al., published in Processings of the MRS (Materials Research Society) International Meeting on Advanced Materials, Vol. 10, Multilayers (May 31-June 1, 1988), a vertical-cavity semiconductor laser structure was described having a multilayered ex situ (non-epitaxially) deposited quarterwave dielectric reflector composed, for example, of alternating layers of TiO.sub.2 /SiO.sub.2 (titanium dioxide/silicon dioxide) located on the top (light-emitting) surface of the laser structure. An Au/SiO.sub.2 (gold/silicon dioxide) ring/electrode reflector was located on the bottom (non-light-emitting) surface thereof. In that way, the top mirror could have higher reflectivity with shorter processing time and with wider optical stop band. To the extent, however, that that laser structure relies upon metallic reflection at the bottom surface, the structure suffers from undesirable optical loss, as well as from relatively low optical reflectivity thereat, typically below about 97 or 98 per centum, as compared with a desired reflectivity in excess of 99 per centum. And to the extent that that laser relies upon reflection by the silicon dioxide layer on the bottom surface, again the attainable optical reflectivity is less than desired.
Therefore, it would be desirable to have a laser structure which mitigates the foregoing disadvantages.